Well, I reread this encounter and decided to go deep and take it as a clue. In particular:
"The fire that shows who we really are and the ink that can write on our black souls."
He seems calm, almost bored, like you're talking about the weather. "They could drive me to ground, but they could never find it, hidden in the stars."
I decided the item / effects that best fit his description was sparkling ink, which gives Burning Vision and Undersea Stalking. And since he was wandering the gallery, I suspected he might of hidden "it" in the Starry Sky painting, which came from the Slags according to the guidebook. Alternatively, I could look in the Slags itself, at ground zero, the other craters, and looking up at the damaged buildings.
Unfortunately there were no results. Sober or etheric, hall of paintings or in the craters, guidebook equipped or not, there were no changes.
I don't have many more ideas, but if anyone wants to get to the Sixth Floor & then drink sparkling ink, well, probably nothing will happen but if anything did it might be the coolest discovery of the year. I'm asking because I don't have a protein analyzer this run, so I can't do it unless someone sends me another vial.
Of course, it's possible it really is a clue but I haven't interpreted it correctly. Is there a less literal interpretation of "fire" and "ink" than the burning vision / octopus ink combo?
EDIT: Forgot to mention I also had Call of the Gift and Hunt for the Gift active, since the swirling maelstrom sketch has a similar tug as the starry sky painting, and they both reference the actual maelstrom.
In the beginning, men were mortal.